This invention pertains to method and apparatus for pulping lignocellulose. It pertains particularly to method and apparatus for pulping lignocellulose in continuously pressurized batch digesters. The invention is applicable particularly to the pulping of wood chips and is described with reference to this application, although it is applicable also to the pulping of other lignocellulose such as bagasse and various vegetable products in the form of small pieces.
The conversion of lignocellulose to fibrous pulps is a sophisticated and highly developed art which supplies a wide variety of cellulosic pulp products to industry on the large commercial
p Of the various well known pulping procedures, those in which the lignocellulose is pulped by cooking with chemicals are the most highly significant. The cooking operation is carried out either batchwise or continuously, depending upon whether batch type or continuous type digesters are employed.
In the batch process, the raw materials are charged into a vessel at atmospheric pressure. The pulping chemicals in solution form are pumped to the digester either during or after the operation by which it is charged with solid lignocellulose.
After the digester has been fully charged at atmospheric pressure, it is sealed and its contents raised to cooking temperature and pressure by the introduction of pressurized steam. The time required to reach cooking temperature (time to temperature) varies normally between 45 minutes and 90 minutes depending primarily upon the availability of high pressure steam. The digester contents are held at the predetermined pressure and temperature (time at temperature) until the lignocellulose has been cooked to the desired degree. The digester then blown. This operation discharges the cellulosic pulp and at the same time reduces the digester to atmospheric pressure.
In the digestion of lignocellulose by the continuous process, on the other hand, the digester is charged continuously with the solid lignocellulose and pulping liquors at a controlled rate and against cooking pressure. The charge progresses continuously through the digester and is discharged continuously therefrom, also at a controlled rate, without loss of digester pressure.
In a comparison of the two, the batch digestion process demonstrates important advantages which have determined its popularity in the paper mills of the world over a long period of time.
Batch digestion apparatus is relatively simple in construction and operation, as well as easy to maintain. It is comparatively inexpensive so that capital investment is relatively low. Elaborate and sensitive control systems are not required. The apparatus is easy to start and stop; easy to fill and empty. Down time is low. Recovery of turpentine, a by-product of increasing value, is high.
Despite these significant advantages, the operation of batch-type digesters is attended by several unfavorable features. Important among these is their high steam requirement stemming from the relatively large volume of digesting liquors which must be used to submerge the chips; the necessity of carrying out heat recovery operations at atmospheric pressure as the digester is emptied; and the necessity of heating the digester shell each time the apparatus is filled with cold chips at atmospheric pressure.
Additionally, batch digesters have the disadvantage that they do not produce pulp of uniform quality. This results from several causes.
A principal cause lies in the impossibility of heating the digester charge uniformly. This is important, because a variance in cooking temperature of as little as 2.degree. F. will result in measurable variation in pulp quality. A digester holds many tons of material. When the temperature of the charge is increased by local application of heat, local elevation of the charge temperature at the points of heat application obviously occurs. The charge approaches uniform temperature throughout only when the digester reaches cooking temperature.
Second, the method used for emptying or blowing a batch digester contributes to non-uniform pulp quality. The vessel conventionally is emptied in a relatively short time through a single pipe of relatively small diameter, as compared with the digester diameter. In a typical installation the blow pipe of a digester having a diameter of 9 to 14 feet may have a diameter of but 6 to 10 inches.
The necessary result is that the thick mass of cooked pulp at a consistency of from 16 to 22% does not discharge from the digester in an orderly fashion. Rather, some of the pulp toward the top of the digester exhausts therefrom in advance of pulp located further down in the digester. Obviously, the latter pulp has been cooked longer than the former pulp with the result that the pulp leaving the digester is of non-uniform quality.
A third factor rendering batch digester pulp non-uniform lies in the difficulty of controlling all of the variables when cooking successive batches of pulp. The chips or other lignocellulose charged to the digester in successive batches are subject to variations in moisture content and density. Such variations will change cooking conditions, even though nearly identical volumes of solid raw material and chemical solutions are fed successively to the digesters. The result is the production of non-uniform pulp over a series of batch digester runs.
A fourth factor affecting pulp uniformity adversely is that the usual variation in chip moisture content from batch to batch results necessarily in a corresponding variation in the amount of condensate formed within the digester. This in turn affects the concentration of the digester liquors in the vicinity of the chips so that in a given time interval a greater or lesser degree of chip cooking will occur.
Yet another disadvantage attending the application of batch digesters is determined by the fact that since the digesters are charged cold with chips in a rigid and unyielding condition, advantage cannot be taken of the compaction factor which potentially is present if the chips were to be in a soft and yielding condition. This decreases the capacity of the digester correspondingly.
The continuous process of digesting cellulosic pulps overcomes many of the foregoing deficiencies which characterize the batch process.
In particular, steam requirements are comparatively low, peak steam loading problems are minimized, and pulp uniformity is improved, as is the pulp compaction factor. Nevertheless, these advantages of the continuous pulp digesting method are over-shadowed by such adverse factors as the complexity and high cost of the continuous apparatus, which necessarily must be employed; increased capital expenditures; increased maintenance; difficulties incurred in starting, stopping, and filling the apparatus; production losses during down time; and low turpentine recovery.
The present invention is designed to provide a batch process for pulping lignocellulose which overcomes the deficiencies of the prior art batch-pulping procedures while retaining their advantages.
It thus is a principal of the present invention to provide a batch procedure for pulping lignocellulose, which process produces a high yield of cellulosic pulp of uniform quality in batch apparatus of simple, easily operated construction having sharply reduced steam requirements even lower than those enjoyed heretofore by the continuous digesters of the prior art.
Another object of the present invention is the provision of a batch procedure for pulping lignocellulose which compensates automatically for variable moisture content of the lignocellulose charge; which has exceptionally high heat recovery; which makes possible an increased recovery of turpentine; which has a high compaction ratio of the chips charged to the digester; and which produces a uniform pulp of high grade.
Still another object of the present invention is the provision of batch lignocellulose pulping apparatus making possible the effectuation of a process, achieving the foregoing objects in batch digesters of the class generally in use with but relatively minor modifications thereof.
The foregoing and other objects of the present invention are achieved by the provision of a batch process for pulping lignocellulose which, basically considered, differs from the conventional batch process in that the digesters are continuously pressurized during their filling, cooking, and discharging cycles.
In carrying out the process, the digester first is brought to pressure by the introduction of steam. The predetermined quantities of wood chips or other lignocellulose and of the white liquor then are introduced into the digester against the steam pressure, together with a variable quantity of black liquor, as required. The feed materials are fed in controlled flows over predetermined time intervals until the calculated quantities have been added. The feed flows then are stopped.
The digester is maintained at cooking pressure and temperature throughout the entire operation. When the material first charged to the digester has been cooked to the desired extent, the discharge flow is initiated and continued at a rate substantially equal to the rate of feed of the raw materials. It then is continued until all of the cooked pulp has been exhausted from the digester. Thus if the cooking cycle comprises one hour, the flow of feed materials may be started and continued for a period of one hour. At the end of this time the digester will be fully charged and the feed flow is stopped. Also, the chips first introduced will have been subjected to cooking conditions for the required one hour period.
The discharge flow of pulp thereupon is started and continued for one hour at substantially the same rate as the feed was introduced into the digester. As a consequence, the last pulp to be discharged from the digester will have been cooked for exactly one hour, as will all intermediate pulp portions.
If it is desired to fill the digester completely in a shorter period, for example 30 minutes, while still retaining a one hour cooking cycle, the flow is started at such a rate as to accomplish this purpose. The flow of feed materials is terminated after 30 minutes and the digester maintained at temperature and pressure for another 30 minutes. At the end of this time the material first introduced will have been cooked exactly one hour. Accordingly, the discharge flow is started and continued at a rate such that the material is completely discharged from the digester in a period of one-half hour.
To expedite this procedure, and to conserve neat, the method is carried on to best advantage using a plurality of digesters in cooperation with each other. One digester is filled as another is emptied, using the same time cycles, and using the hot black liquor from an emptying digester as part of the charge for a filling digester, all the while maintaining both systems at full operating pressure, as will be explained in greater detail hereinafter.
To accomplish this result, batch digestion apparatus is provided wherein a plurality of digesters, at least two, and preferably four, are arranged in a group and provided with means for feeding them sequentially from separate or common sources. Suitable piping is provided for introducing steam, pulping chemicals and digestion liquor, both fresh and spent, into the digesters as needed. Furthermore, each digester at its outlet end is provided with means for introducing spent displacement liquor in such a manner as to displace the hot black liquor for heat recovery and reuse while diluting the pulp for discharge from the digester.